X-ray imaging techniques are essential diagnostic tools in medicine, biology, material research, non-destructive testing and quality control. For a long time, X-ray sensitive films were the preferred means for the acquisition of two-dimensional images of transmitted local X-ray intensity. These X-ray films are increasingly replaced by digital X-ray imaging systems, where the picture is created directly as an electronic image of the X-ray intensity distribution, and no chemical development of a photographic film is required. Such digital detectors can be categorized into three classes:
Image plates, also called CR (Computed Radiography) systems, consist of an X-ray sensitive layer, in which an incident X-ray photon interacts with photostimulable crystals in the CR layers by locally transferring electrons into a semi-stable excited state. Using a scanning laser beam, these states are de-excited in a process called photostimulated luminescence, in which the locally stored energy is released as light flashes. These light flashes can be detected by sensitive photodetectors such as photomultiplier tubes or semiconductor photosensors. Because the X-ray images in a CR system do not appear directly as analog or digital data but rather need an additional readout procedure, the use of CR systems in currently waning.
A second category of X-ray imaging systems is called DR (Direct Radiography). Incident X-rays directly interacting in the bulk of a semiconductor, where they generate mobile electron-hole pairs. One or both types of generated charge carriers are collected and subsequently read out with electronic circuits for the detection of charge packets. The most common semiconductor employed for technical applications is silicon. Therefore, the first DR systems used a silicon layer to absorb and convert the X-rays into charge packets. Unfortunately, silicon has a low atomic number, and it is therefore a comparably inefficient absorber for X-rays. Hence, silicon can only be used as a detector material for DR using X-ray energies of up to about 30 keV, which is insufficient for many medical, technical and non-destructive testing applications. Recently, other semiconductor materials such as germanium, selenium or cadmium-telluride are being used as X-ray detectors because due to their comparatively high atomic numbers they are much better absorbers. Unfortunately, these semiconductor materials are also much more difficult to grow in good quality. Furthermore, since all of these detector materials are read out with silicon-based integrated electronic charge detection circuits, they require a complicated and expensive bump-bonding process to connect the absorption layers with the read-out devices. This difficult fabrication process contributes to the comparably high price of such X-ray imaging systems.
Today, the vast majority of X-ray imaging systems make use of a scintillating layer which converts the incident X-ray photons into pulses of visible light, and an optically coupled array of photodiodes collects the visible light and converts it into electrical signals. Such systems are called I-DR (Indirect Digital Radiography) systems. The required photodiodes can be made of amorphous silicon (thin-film-transistor technology) or of crystalline semiconductors such as silicon, which can also be integrated into ASICs (Application Specific Integrated Circuits).
The advantage of such I-DR systems is that they rely on established and mature fabrication technologies and are therefore very cost-effective. However, the generation of light in a homogeneous scintillator screen is not directed, and therefore it is distributed over a number of photosensor elements in the image sensor which limits the spatial resolution. This can be overcome by the creation of structured scintillation screens with columnar pixels, each covered with highly reflecting material, as disclosed by S. Petterson et al. in the European Patent EP 1,161,693 B1, “X-Ray Pixel Detector and Fabrication Method”. However, the preferred image sensor for such a scintillating device is a CCD (Charge-Coupled Device) which depends on clocked charge transport for its operation. As a consequence, it is not possible with such a system to detect the arrival time of a single X-ray photon.
This deficiency can be partly overcome with the invention described by C. S. Levin et al in the U.S. Pat. No. 6,114,703, “High Resolution Scintillation Detector With Semiconductor Readout”. This patent discloses the use of columnar parallelepiped scintillation crystals, whose long faces are optically coupled to semiconductor photosensors. Since the area of each photosensor is the same as the area of the crystal's long face, the scintillation light is transmitted homogeneously and completely from the crystal into the photosensor. However, it is known that the detection charge noise of an electronic charge detection circuit is proportional to the effective capacitance of the sense node, as explained for example by P. Seitz and A. J. P. Theuwissen (Eds.), in “Single-Photon Imaging”, Springer 2011. For this reason, a large surface of the photosensor device is detrimental to the detection of light pulses with low amplitude, i.e. for the sensitive detection of single X-ray photons with energies in the range of a few 10 keV. For this reason, the X-ray detector described in U.S. Pat. No. 6,114,703 is targeted mainly to applications in PET (Positron Emission Tomography) where the incident X-ray photons have very high energies exceeding 500 keV.
This problem of reduced sensitivity is partially overcome with the X-ray detector architecture described by A. Balan et al. in the United States Patent Application US 2009/0008564 A1, “Modular X-Ray Detector With Single Photon Counting, Energy Sensitivity and Integration Capabilities”. By transmitting the scintillation light of a structured scintillation screen through the short face of the columnar elements, and by optically coupling the scintillator screen to a photodiode of small area, the effective noise of the charge detection circuit is reduced. However, the typical area of an X-ray pixel is of the order of 100×100 micrometers, which is significantly larger than the less than 5×5 micrometers encountered in state-of-the-art low noise semiconductor image sensors. As a consequence of this large photosensing area, relatively high charge noise is encountered in these state-of-the-art X-ray detectors; in US 2009/0008564 A1, typical charge noise standard-deviation values of 70-300 electrons are quoted. This is considerably too high for the reliable detection of single X-ray photons in the medically and technically interesting energy range of 10-300 keV, when additional information about the energy of the detected X-ray photon is required simultaneously.